Showing papers in "Plasma Sources Science and Technology in 2013"

Transport mechanisms of metastable and resonance atoms in a gas discharge plasma

Abstract: Atoms in electronically excited states are of significant importance in a large number of different gas discharges. The spatio-temporal distribution particularly of the lower excited states, the metastable and resonance ones, influences the overall behavior of the plasma because of their role in the ionization and energy budget. This article is a review of the theoretical and experimental studies on the spatial formation and temporal evolution of metastable and resonance atoms in weakly ionized low-temperature plasmas. Therefore, the transport mechanisms due to collisional diffusion and resonance radiation are compared step by step. The differences in formation of spatio-temporal structures of metastable and resonance atoms in plasmas are attributed to these different transport mechanisms. The analysis is performed by obtaining solutions of the diffusion and radiation transport equations. Solutions of stationary and non-stationary problems by decomposition over the eigenfunctions of the corresponding operators showed that there is, on the one hand, an effective suppression of the highest diffusion modes and, on the other hand, a survival of the highest radiation modes. The role of the highest modes is illustrated by examples. In addition, the differences in the Green functions for the diffusion and radiation transport operators are discussed. Numerical methods for the simultaneous solution of the balance equations for metastable and resonance atoms are proposed. The radiation transport calculations consider large absorption coefficients according to the Lorentz contour of a spectral line. Measurements of the distributions of metastable and resonance atoms are reviewed for a larger number of discharge conditions, i.e. in the positive column plasma, afterglow plasma, constricted pulsed discharge, stratified discharge, magnetron discharge, and in a discharge with a cathode spot.

Tritium inventory control during ITER operation under carbon plasma-facing components by nitrogen-based plasma chemistry: a review

Abstract: In spite of being highly suited for advanced plasma performance operation of tokamaks, as demonstrated over at least two decades of fusion plasma research, carbon is not currently considered as an integrating element of the plasma-facing components (PFCs) for the active phase of ITER. The main reason preventing its use under the very challenging scenarios foreseen in this phase, with edge-localized modes delivering several tens of MW m−2 to the divertor target every second or less, is the existing concern about reaching the tritium inventory value of 1000 g used in safety assessments in a time shorter than the projected lifetime of the divertor materials eroded by the plasma, set at 3000 shots. Although several mechanisms of tritium trapping in carbon components have been identified, co-deposition of the carbon radicals arising from chemically eroded chlorofluorocarbons in remote areas appears to play a dominant role. Several possible ways to keep control of the tritium build-up during the full operation of ITER have been put forward, mostly based on the periodic removal of the co-deposits by chemical (thermo-oxidation, plasma chemistry) or physical (laser, flash lamps) methods. In this work, we review the techniques for the inhibition and removal of tritium-rich co-deposits based on the strong chemical reactivity of some N-bearing molecules with carbon. The integration of these techniques into a possible scheme for tritium inventory control in the active phase of ITER under carbon-based PFCs with minimum down-time is discussed and the existing caveats are addressed.

Interacting kinetics of neutral and ionic species in an atmospheric-pressure helium-oxygen plasma with humid air impurities

Abstract: We unravel the complex chemistry in both the neutral and ionic systems of a radio-frequency-driven atmospheric-pressure plasma in a helium?oxygen mixture (He?0.5% O2) with air impurity levels from 0 to 500?ppm of relative humidity from 0% to 100% using a zero-dimensional, time-dependent global model. Effects of humid air impurity on absolute densities and the dominant production and destruction pathways of biologically relevant reactive neutral species are clarified. A few hundred ppm of air impurity crucially changes the plasma from a simple oxygen-dependent plasma to a complex oxygen?nitrogen?hydrogen plasma. The density of reactive oxygen species decreases from 1016 to 1015?cm?3, which in turn results in a decrease in the overall chemical reactivity. Reactive nitrogen species (1013?cm?3), atomic hydrogen and hydroxyl radicals (1011?1014?cm?3) are generated in the plasma. With 500?ppm of humid air impurity, the densities of positively charged ions and negatively charged ions slightly increase and the electron density slightly decreases (to the order of 1011?cm?3). The electronegativity increases up to 2.3 compared with 1.5 without air admixture. Atomic hydrogen, hydroxyl radicals and oxygen ions significantly contribute to the production and destruction of reactive oxygen and reactive nitrogen species.

Absolute OH density measurements in the effluent of a cold atmospheric-pressure Ar–H2O RF plasma jet in air

Abstract: Absolute OH densities are obtained in a radio-frequency-driven Ar–H2O atmospheric-pressure plasma jet by laser-induced fluorescence (LIF), calibrated by Rayleigh scattering and by UV broadband absorption. The measurements are carried out in ambient air and the effect of air entrainment into the Ar jet is measured by analyzing the time-resolved fluorescence signals. The OH densities are obtained for different water vapor concentrations admixed to the Ar and as a function of the axial distance from the nozzle. A sensitivity analysis to deduce the accuracy of the model-calculated OH density from the LIF measurement is reported. It is found that the UV absorption and the LIF results correspond within experimental accuracy close to the nozzle and deviate in the far effluent. The possible reasons are discussed. The OH densities found in the plasma jet are in the range (0.1–2.5) × 1021 m−3 depending on the water concentration and plasma conditions.

A benchmark study of a capacitively coupled oxygen discharge of the oopd1 particle-in-cell Monte Carlo code

Abstract: The oopd1 particle-in-cell Monte Carlo collision (PIC-MCC) code is used to simulate a capacitively coupled discharge in oxygen. oopd1 is a one-dimensional object-oriented PIC-MC code in which the model system has one spatial dimension and three velocity components. It contains a model for planar geometry and will contain models for cylindrical and spherical geometries, and replaces the xpdx1 series, which is not object-oriented. The oopd1 also allows for different weights of simulation particles and relativistic treatment of electrons. The revised oxygen model includes, in addition to electrons, the oxygen molecule in the ground state, the oxygen atom in the ground state, the negative ion O− and the positive ions O+ and . The cross sections for the collisions among the oxygen species have been significantly revised from earlier work using the xpdp1 code and the electron kinematics have been enhanced. Here we make a benchmark study and compare the oopd1 code to the well-established planar xpdp1 code and discuss the differences using a limited cross section set with ions, O− ions and electrons as the charged particles. We compare the electron energy distribution function, the electron temperature profile, the density profiles of charged particles and electron heating rates for a capacitively coupled oxygen discharge at 50 mTorr with electrode separation of 4.5 cm. Then we explore the effect of adding O atoms and O+ ions on the overall discharge.

Atomic oxygen TALIF measurements in an atmospheric-pressure microwave plasma jet with in situ xenon calibration

Abstract: Two-photon absorption laser-induced fluorescence (TALIF) is used to measure the absolute density of atomic oxygen (O) in a coaxial microwave jet in ambient air at atmospheric pressure, operated with a mixture of He and a few per cent of air. The TALIF signal is calibrated using a gas mixture containing Xe. A novel method to perform calibration in situ, at atmospheric pressure, is introduced. The branching ratios of several Xe mixtures are reported, to enable us to perform the Xe calibration without the need for a vacuum vessel. The O densities are measured spatially resolved, and as a function of admixed air to the He, and microwave power. The electron density and temperatures are measured using Thomson scattering, and the N2 and O2 densities are measured using Raman scattering. O densities are found to have a maximum of (4?6)???1022?m?3, which indicate that O2 is close to fully dissociated in the plasma. This is confirmed by the Raman scattering measurements. O is found to recombine mainly into species other than O2 in the afterglow, which is suggested to consist of O3 and oxidized components of NO.

On sheath energization and Ohmic heating in sputtering magnetrons

Abstract: In most models of sputtering magnetrons, the mechanism for energizing the electrons in the discharge is assumed to be sheath energization. In this process, secondary emitted electrons from the cathode surface are accelerated across the cathode sheath into the plasma, where they either ionize directly or transfer energy to the local lower energy electron population that subsequently ionizes the gas. In this work, we present new modeling results in support of an alternative electron energization mechanism. A model is experimentally constrained, by a fitting procedure, to match a set of experimental data taken over a large range in discharge powers in a high-power impulse magnetron sputtering (HiPIMS) device. When the model is matched to real data in this way, one finding is that the discharge can run with high power and large gas rarefaction without involving the mechanism of secondary electron emission by twice-ionized sputtered metal. The reason for this is that direct Ohmic heating of the plasma electrons is found to dominate over sheath energization by typically an order of magnitude. This holds from low power densities, as typical for dc magnetrons, to so high powers that the discharge is close to self-sputtering, i.e. dominated by the ionized vapor of the sputtered gas. The location of Ohmic heating is identified as an extended presheath with a potential drop of typically 100–150 V. Such a feature, here indirectly derived from modeling, is in agreement with probe measurements of the potential profiles in other HiPIMS experiments, as well as in conventional dc magnetrons.

Effects of pulse voltage rise rate on velocity, diameter and radical production of an atmospheric-pressure streamer discharge

Abstract: The effect of pulse rise rate on a streamer discharge is investigated through both experiments and simulations. Pulsed voltages with a pulse rise rate of 0.11–0.52 kV ns−1 are applied to point-to-plane electrode configurations, and the effects are observed from ICCD photographs. The streamer emission of light is simulated by a previously developed two-dimensional streamer simulation model, and the simulation results are compared with experimental results. The results show that as the pulse rise rate is decreased, there is a decrease in the discharge current, velocity of the primary streamer, diameter of the streamer channel and emission length of the secondary streamer. The simulated reduced electric field of the primary streamer head remains constant and does not depend on the pulse rise rate. The simulated temporal variations of O and OH radical production show that almost the same number of the radicals are produced in the primary streamer, regardless of the pulse rise rate. However, the radical production in the secondary streamer decreases as the pulse rise rate decreases. Therefore, the pulse rise rate affects the ratio of radical production in the primary streamer to that in the secondary streamer.

A study on the temporally and spatially resolved OH radical distribution of a room-temperature atmospheric-pressure plasma jet by laser-induced fluorescence imaging

Abstract: In this paper, the time and spatially resolved OH distribution of a room-temperature atmospheric-pressure plasma jet is investigated using a laser-induced fluorescence (LIF) method. The plasma jet is generated in room air by applying a nanosecond pulsed high voltage onto a ceramic tube with helium gas flow. It is found that, before the plasma 'bullet' propagates through the region detected by the laser, there are low OH LIF signals, which are from the OH left from previous discharge pulses. After the propagation of the primary plasma 'bullet' (generated by the primary discharge at the rising edge of the voltage pulse) through the detected region, the OH LIF signals are more than doubled. Furthermore, after the propagation of the secondary plasma 'bullet' (generated by the secondary discharge at the falling edge of the voltage pulse) through the detected region, the OH LIF signals are doubled again. Then, after the voltage pulse, the OH LIF signals decay slowly until about 120 µs. Starting from about 120 µs after the voltage pulse, the OH LIF signals have a third increase; its peak value is more than doubled. Detailed investigations find that this is due to the gas flow, which blows the OH generated inside the discharge tube to the detected region. In addition, spatially resolved OH LIF signals show that the signal intensity is stronger on both edges, which gives rise to the donut shape of the OH distribution. Further studies reveal that this might be due to the interaction of the plasma plume with the surrounding water vapor in the air.

Pulsed nanosecond discharge development in liquids with various dielectric permittivity constants

Abstract: The dynamics of pulsed nanosecond discharge development in liquid water, ethanol and hexane were investigated experimentally. High-voltage pulses with durations of 20 and 60 ns and amplitudes of 6–60 kV were used for discharge initiation. It is shown that the dynamics of discharge formation fundamentally differ between liquids with low and high dielectric permittivity coefficients. In water (high permittivity), two phases were observed in the process of discharge development. The first phase is connected with electrostriction compression of the media near the needle tip and the formation of a rarefaction wave in the surrounding liquid. The second phase (the discharge phase) has a pronounced start delay, which depends on the voltage of the high-voltage electrode. Thus, at low voltages, the pulse length is insufficient for the initiation of the discharge, and the process consists of the first phase only, i.e. the formation of an electrostriction rarefaction wave. At higher voltages, the discharge start delay time decreases rapidly, and the discharge commences simultaneously with the formation of hydrodynamic perturbations by the electrostriction forces present in the media. Shadowgraphic laser visualization of the process demonstrates the transition from a pure hydrodynamic density perturbation in the rarefaction wave to a developed streamer-leader process with a strong energy release in the channels and the formation of strong shock waves around the channels. Unlike in water, the first phase is essentially non-existent in liquids with low dielectric permittivity coefficients because of the small electrostriction forces and the low intensity of the rarefaction wave that is formed. The second phase in the process (discharge) begins at significantly higher voltages on the high-voltage electrode, immediately leading to the long branched structure of the streamer-leader flash. The difference in the nanosecond discharge development in liquid dielectrics may be explained by the formation of micro-discontinuities in the media during the electrostriction compression/rarefaction stage in liquids with high dielectric permittivities.

Formation of self-organized anode patterns in arc discharge simulations

Abstract: Pattern formation and self-organization are phenomena commonly observed experimentally in diverse types of plasma systems, including atmospheric-pressure electric arc discharges. However, numerical simulations reproducing anode pattern formation in arc discharges have proven exceedingly elusive. Time-dependent three-dimensional thermodynamic non-equilibrium simulations reveal the spontaneous formation of self-organized patterns of anode attachment spots in the free-burning arc, a canonical thermal plasma flow established by a constant dc current between an axi-symmetric electrode configuration in the absence of external forcing. The number of spots, their size and distribution within the pattern depend on the applied total current and on the resolution of the spatial discretization, whereas the main properties of the plasma flow, such as maximum temperatures, velocity and voltage drop, depend only on the former. The sensibility of the solution to the spatial discretization stresses the computational requirements for comprehensive arc discharge simulations. The obtained anode patterns qualitatively agree with experimental observations and confirm that the spots originate at the fringes of the arc–anode attachment. The results imply that heavy-species–electron energy equilibration, in addition to thermal instability, has a dominant role in the formation of anode spots in arc discharges.

Electron densities and energies of a guided argon streamer in argon and air environments

Abstract: In this study we report the temporally and spatially resolved electron densities and mean energies of a guided argon streamer in ambient argon and air obtained by Thomson laser scattering. The plasma is driven by a positive monopolar 3.5 kV pulse, with a pulse width of 500 ns and a frequency of 5 kHz which is synchronized with the high repetition rate laser system. This configuration enables us to use the spatial and temporal stability of the guided streamer to accumulate a multitude of laser/plasma shots by a triple grating spectrometer equipped with an ICCD camera and to determine the electron parameters. We found a strong initial ne-overshoot with a maximum of 7 × 1019 m−3 and a mean electron energy of 4.5 eV. This maximum is followed by a fast decay toward the streamer channel. Moreover, a 2D distribution of the electron density is obtained which exhibits a peculiar mushroom-like shape of the streamer head with a diameter significantly larger than that of the emission profile. A correlation of the width of the streamer head with the expected pre-ionization channel is found.

Measurement of hydroxyl radical (OH) concentration in an argon RF plasma jet by laser-induced fluorescence

Abstract: The concentration of hydroxyl (OH) radicals in a plasma pencil, an atmospheric RF plasma jet ignited in argon, was measured by laser-induced fluorescence calibrated by Rayleigh scattering on ambient air. A suitable excitation scheme for this discharge was suggested based on laser excitation to the lowest vibrational level of the A 2Σ+ state. Effects of spectral overlap between the laser and absorption line, fluorescence saturation, temporal evolution of fluorescence radiation, rotational energy transfer and rotational distribution on the diagnostic method were analysed. The maximum OH concentration was approximately 5 × 1020 m−3. The maximum was reached when the measurement point localized in the effluent of the plasma pencil was inside but close to the tip of the visible active discharge.

Electron heating and control of ion properties in capacitive discharges driven by customized voltage waveforms

Abstract: We investigate the electron heating dynamics in capacitively coupled radio frequency plasmas driven by customized voltage waveforms and study the effects of modifying this waveform and the secondary electron emission coefficient of the electrodes on the spatio-temporal ionization dynamics by particle-in-cell simulations. We demonstrate that changes in the electron heating dynamics induced by voltage waveform tailoring strongly affect the dc self-bias, the ion flux, Γi, and the mean ion energy, 〈Ei〉, at the electrodes. The driving voltage waveform is customized by adding N consecutive harmonics (N ⩽ 4) of 13.56 MHz with specific harmonics' amplitudes and phases. The total voltage amplitude is kept constant, while modifying the number of harmonics and their phases. In an argon plasma, we find a dc self-bias, η, to be generated via the electrical asymmetry effect for N ⩾ 2. η can be controlled by adjusting the harmonics' phases and is enhanced by adding more consecutive harmonics. At a low pressure of 3 Pa, the discharge is operated in the α-mode and 〈Ei〉 can be controlled by adjusting the phases at constant Γi. The ion flux can be increased by adding more harmonics due to the enhanced electron-sheath heating. 〈Ei〉 does not remain constant as a function of N at both electrodes due to a change in η. These findings verify previous results of Lafleur et al. At a high pressure of 100 Pa and using a high secondary electron emission coefficient of γ = 0.4, the discharge is operated in the γ-mode and mode transitions are induced by changing the driving voltage waveform. Due to these mode transitions and the specific ionization dynamics in the γ-mode, Γi is no longer constant as a function of the harmonics' phases and decreases with increasing N.

Elaboration of collisional–radiative models for flows related to planetary entries into the Earth and Mars atmospheres

Abstract: The most relevant way to predict the excited state number density in a nonequilibrium plasma is to elaborate a collisional–radiative (CR) model taking into account most of the collisional and radiative elementary processes. Three examples of such an elaboration are given in this paper in the case of various plasma flows related to planetary atmospheric entries. The case of theoretical determination of nitrogen atom ionization or recombination global rate coefficients under electron impact is addressed first. The global rate coefficient can be implemented in multidimensional computational fluid dynamics calculations. The case of relaxation after a shock front crossing a gas of N2 molecules treated in the framework of the Rankine–Hugoniot assumptions is also studied. The vibrational and electronic specific CR model elaborated in this case allows one to understand how the plasma reaches equilibrium and to estimate the role of the radiative losses. These radiative losses play a significant role at low pressure in the third case studied. This case concerns CO2 plasma jets inductively generated in high enthalpy wind tunnels used as ground test facilities. We focus our attention on the behaviour of CO and C2 electronic excited states, the radiative signature of which can be particularly significant in this type of plasma. These three cases illustrate the elaboration of CR models and their coupling with balance equations.

Resonant vibrational-excitation cross sections and rate constants for low-energy electron scattering by molecular oxygen

Abstract: Resonant vibrational-excitation cross sections and rate constants for electron scattering by molecular oxygen are presented. Transitions between all 42 vibrational levels of are considered. Molecular rotations are parametrized by the rotational quantum number J, which is considered in the range 1–151. The lowest four resonant states of , 2Πg, 2Πu, and are taken into account. The calculations are performed using the fixed-nuclei R-matrix approach to determine the resonance positions and widths, and the boomerang model to characterize the nuclei motion. Two energy regions below and above 4 eV are investigated: the first one is characterized by sharp structures in the cross section and the second by a broad resonance peaked at 10 eV. The computed cross sections are compared with theoretical and experimental results available in the literature for both energy regions, and are made available for use by modelers. The effect of including rotational motion is found to be non-negligible.

Cavitation in the vicinity of the high-voltage electrode as a key step of nanosecond breakdown in liquids

Abstract: Fast shadowgraphy of nanosecond discharge in liquids with different dielectric permittivity, namely in water, ethanol and n-pentane, has been performed. Formation of a gas cavity at a nanosecond time scale was observed as a pre-breakdown phenomenon at amplitudes of the high-voltage pulse close to the breakdown threshold. This phenomenon is considered as a possible key step of high-voltage breakdown in polar liquids.

Controlling the electron energy distribution function of electron beam generated plasmas with molecular gas concentration: I. Experimental results

Abstract: This paper, the first in a series of two, presents experimental results demonstrating the control of electron and ion energy distribution functions in electron beam generated processing plasmas by adding trace concentrations of N2 to an Ar background. Measurements of the electron energy distribution function, f0(E), are performed using a Langmuir probe while measurements of the Ar ion energy distribution function are performed using an energy-resolved mass spectrometer. The experimental results agree with modeling results, described in part II of this work, which indicate that inelastic electron collisions with nitrogen molecules provide an energy sink that can be exploited to control the electron energy distribution function.

Comparison of sinusoidal and pulsed-operated dielectric barrier discharges in an O 2 /N 2 mixture at atmospheric pressure

Abstract: Experiments on the spatial and temporal structure of the breakdown process of sinusoidal- and pulsed-operated dielectric barrier microdischarges (MDs) are compared. Three different waveforms are considered: a sinusoidal waveform at 20 kHz and pulsed-bipolar and unipolar-voltage profiles at 10 kHz with varying duty cycles (asymmetric pulse). Electrical data and simultaneous streak and iCCD images of individual MDs in dielectric barrier discharges (DBDs) with 1 mm gap in a gas mixture of 0.1 vol% O2 in N2 at atmospheric pressure are recorded. For sinusoidal-operated DBDs there are no significant differences between the MDs at positive and negative half-periods. Sinusoidal operation corresponds to pulsed-bipolar operation with symmetrical pulses, but with lower streamer velocities and different spatio-temporal emission distribution. The development of pulsed-driven MDs is determined by the voltage between both electrodes and not by the polarity of the driven electrode, resulting in nearly the same behavior of bipolar- and unipolar-pulsed-driven MDs. DBDs operated with asymmetric pulses show a significant difference in the spatial structure and in the temporal behavior between the rising and falling slopes of the high voltage pulse.

Self-organized pattern formation in helium dielectric barrier discharge cryoplasmas

Abstract: Self-organized patterns appear in many biological, chemical and physical systems, including electric discharges. Under certain conditions, self-organized patterns also form in plasmas generated below room temperature. These so-called cryoplasmas have also shown promise for low-damage materials processing; however, the underlying mechanisms and experimental conditions that lead to either uniform discharges or those containing self-organized patterns are still not understood completely. Here, we investigated the formation and dynamics of self-organized patterns in dielectric barrier cryoplasmas generated at plasma gas temperatures ranging from 264 down to 7 K at a constant gas density ρ = 5 × 1019 cm−3. The electrode gap was 0.15 mm and the cryoplasmas were generated at voltages between 0.8 and 1.5 kV, at frequencies ranging from 20 to 30 kHz. The discharges were characterized by time-resolved imaging, optical emission spectroscopy and current–voltage measurements. For temperatures down to 250 K, the discharges are uniform, whereas between 250 and about 140 K, self-organized, bright filamentary patterns form. Below that temperature, the discharge regime changes again to a uniform glow and for temperatures below 20 K, different types of discharges—uniform, but also self-organized dark solitons and bright stripe patterns—are observed. The cryoplasmas show current–voltage characteristics that are similar to atmospheric pressure glow discharges and the different types of uniform or self-organized discharges are suggested to be caused by the disappearance of impurities in the plasma as the temperature is lowered, and changes in the mobilities of ion species and surface charges.

Three-dimensional structure of the extraction region of a hybrid negative ion source

Abstract: A self-consistent three-dimensional particle-based model of the source extraction–acceleration transition region of a surface-produced negative ion source is developed. Some considerations are advanced on the characteristic of negative ion transport: it is purely electrostatic while collision-induced (charge exchange with atoms) and magnetic-induced (ion gyration around the filter field) transport contributions play no relevant role in H− extraction. In fact, the calculations presented here indicate that the key point is the penetration of the extraction grid field inside the plasma grid collar and the source region, which helps in removing the negative ions produced on the surface. This study suggests that the best plasma grid shape is characterized so as to allow the extraction field to arrive directly on the surface-emitting H− ions and that the best aperture size is directly related to the particular shape used.

A study of helium atmospheric-pressure guided streamers for potential biological applications

Abstract: The origin of differences in the rotational temperatures of various molecules and ions ( (B), OH(A) and N2(C)) is studied in helium atmospheric-pressure guided streamers. The rotational temperature of (B) is room temperature. It is estimated from the emission band of the first negative system at 391.4?nm, and it is governed by the temperature of N2(X) in the surrounding air. N2(X) is ionized by direct electron impact in the outer part of the plasma. (B) is deactivated by collisions with N2 and O2. The rotational temperature of OH(A), estimated from the OH band at 306.4?nm, is slightly higher than that of (B). OH(A) is excited by electron impact with H2O during the first 100?ns of the applied voltage pulse. Next, OH(A) is produced by electron impact with OH(X) created by the quenching of OH(A) by N2 and O2. H2O diffuses deeper than N2 into the plasma ring and the rotational temperature of OH(A) is slightly higher than that of (B). The rotational temperature of N2(C), estimated from the emission of the second positive system at 315.9?nm, is governed by its collisions with helium. The gas temperature of helium at the beginning of the pulse is predicted to be several hundred kelvin higher than room temperature.

Ozone kinetics in low-pressure discharges: vibrationally excited ozone and molecule formation on surfaces

Abstract: A combined experimental and modeling investigation of the ozone kinetics in the afterglow of pulsed direct current discharges in oxygen is carried out. The discharge is generated in a cylindrical silica tube of radius 1?cm, with short pulse durations between 0.5 and 2?ms, pressures in the range 1?5?Torr and discharge currents ?40?120?mA. Time-resolved absolute concentrations of ground-state atoms and ozone molecules were measured simultaneously in situ, by two-photon absorption laser-induced fluorescence and ultraviolet absorption, respectively. The experiments were complemented by a self-consistent model developed to interpret the results and, in particular, to evaluate the roles of vibrationally excited ozone and of ozone formation on surfaces. It is found that vibrationally excited ozone, , plays an important role in the ozone kinetics, leading to a decrease in the ozone concentration and an increase in its formation time. In turn, the kinetics of is strongly coupled with those of atomic oxygen and O2(a?1?g) metastables. Ozone formation at the wall does not contribute significantly to the total ozone production under the present conditions. Upper limits for the effective heterogeneous recombination probability of O atoms into ozone are established.

Simulation study of stochastic heating in single-frequency capacitively coupled discharges with critical evaluation of analytical models

Abstract: Stochastic heating is an important phenomenon in low-pressure radio-frequency capacitive discharges. Recent theoretical work on this problem using several different approaches has produced results that are broadly in agreement insofar as scaling with the discharge parameters is concerned, but there remains some disagreement in detail concerning the absolute size of the effect. In this paper we investigate the dependence of stochastic heating on various discharge parameters by scaling these parameters with the help of particle-in-cell (PIC) simulation. The analytical models are satisfactory for intermediate current density amplitude (or control parameter H) and in agreement with PIC results. However, for higher (or higher H), new physical effects appear (such as field reversal, electron trapping, reflection of ions, etc) and the simulation results deviate from existing analytical models. In most experiments, but not all, H ∼ 5 and these effects are not observed there. On the other hand, for lower (or lower H) again the simulation results deviate from analytical models. So we have produced a relatively extensive set of simulation data that may be used to validate theories over a wide range of parameters. The dependence of stochastic heating on applied frequency is also investigated here with the help of the self-consistent PIC method.

Modelling of OH production in cold atmospheric-pressure He–H2O plasma jets

Abstract: Results of the modelling of OH production in the plasma bullet mode of cold atmospheric-pressure He–H2O plasma jets are presented. It is shown that the dominant source of OH molecules is related to the Penning and charge transfer reactions of H2O molecules with excited and charged helium species produced by guided streamers (plasma bullets), in contrast to the case of He–H2O glow discharges where OH production is mainly due to the dissociation of H2O molecules by electron impact.

Line-ratio determination of atomic oxygen and ${\rm N}_2(A\,{}^3\Sigma_{\rm u}^+)$ metastable absolute densities in an RF nitrogen late afterglow

Abstract: Optical emission spectroscopy line-ratio methods are developed in order to estimate the absolute densities of nitrogen and oxygen atoms and metastable N2(A) molecules in the nitrogen late afterglow of an RF discharge, operating at p?=?8?Torr, Q?=?1?slm and P?=?100?W, in what constitutes an extension of the typical domain of application of these methods. [N] is obtained from the first positive (1+) emission with calibration by NO titration, [O] from the ratio of the NO? to 1+ bands, and [N2(A)] from the ratios of (i) the NO? and NO? bands, (ii) the second positive (2+) and NO? bands and (iii) the 1+ and 2+ bands. In addition to the determination of the N, O and N2(A) absolute densities, the present investigation gives an indication on the order of magnitude of the rate coefficient of the very important reaction N2(X, v???13)?+?O???NO?+?N at room temperature.

Hydrogen production from alcohol reforming in a microwave ‘tornado’-type plasma

Abstract: In this work, an experimental investigation of microwave plasma-assisted reforming of different alcohols is presented. A microwave (2.45 GHz) 'tornado'-type plasma with a high-speed tangential gas injection (swirl) at atmospheric pressure is applied to decompose alcohol molecules, namely methanol, ethanol and propanol, and to produce hydrogen-rich gas. The reforming efficiency is investigated both in Ar and Ar+ water vapor plasma environments. The hydrogen yield dependence on the partial alcohol flux is analyzed. Mass spectrometry and Fourier transform infrared spectroscopy are used to detect the outlet gas products from the decomposition process. Hydrogen, carbon monoxide, carbon dioxide and solid carbon are the main decomposition by-products. A significant increase in the hydrogen production rate is observed with the addition of a small amount of water. Furthermore, optical emission spectroscopy is applied to detect the radiation emitted by the plasma and to estimate the gas temperature and electron density.

Evaluation of precursor evaporation in Si nanoparticle synthesis by inductively coupled thermal plasmas

Abstract: The evaporation of a micro-sized silicon solid precursor in a laboratory scale inductively coupled thermal plasma system for nanoparticle synthesis is investigated numerically using a customized version of the commercial CFD code ANSYS FLUENT?. Two turbulence models?the standard k?? and the Reynolds stress model?and two different models for the computation of vapour production from the heated precursor?evaporation at boiling point and vaporization driven by vapour concentration gradients?are compared. The choice of the turbulence model can considerably influence the estimation of vapour production because plasma temperature reduction by plasma?particle heat exchange is increased when the flow in the torch region is predicted to be laminar, whereas the choice of the model for particle evaporation may be critical when the plasma temperature is decreased by plasma?particle heat exchange to values close to the boiling point of the material treated.

Simulation of the stable ?quasi-periodic? glow regime of a nanosecond repetitively pulsed discharge in air at atmospheric pressure

Abstract: This paper presents simulations of the dynamics of nanosecond repetitively pulsed discharges between two point electrodes in atmospheric pressure air at 300 and 1000?K. At 300?K, the preionization left by successive discharges at the end of interpulses mainly consists of positive and negative ions with a density of about 109?cm?3 for a repetition frequency of 10?kHz. When photoionization is taken into account with a level of seed charges of about 109?cm?3, the dynamics and the characteristics of the discharge during a voltage pulse are shown to depend only weakly on the nature of negative seed charges (electrons or ions). At 1000?K, the preionization left by successive discharges at the end of interpulses consists of positive and negative ions and electrons with a density of about 1010?cm?3 for a repetition frequency of 10?kHz. Simulation results show that the dynamics and the characteristics of the discharge during a voltage pulse remain rather close whatever the preionization level considered in the range 109?1011?cm?3, corresponding to nanosecond repetitively pulsed discharges in the frequency range 1?100?kHz. The simulation of several consecutive nanosecond voltage pulses at a frequency of 10?kHz shows that, at 1000?K, the discharge can reach in a few voltage pulses a stable ?quasi-periodic? glow regime in agreement with experiments. Finally, the influence of an external air flow aligned with the electrode axis on the conditions to obtain a stable ?quasi-periodic? glow regime is discussed.

Electron flow properties in the far-field plume of a Hall thruster

Abstract: The plasma properties were investigated in the far-field plume of a 1.5 kW class Hall thruster using a single, cylindrical Langmuir probe. The plasma potential, the electron temperature and the electron density were measured at 191 positions, providing a detailed map of the plume pattern. This map shows that the plasma plume of a Hall thruster is an expanding jet that is symmetric about the thruster axis. The large data set was also used for a detailed analysis of the electron flow properties. This analysis reveals that the plasma plume of a Hall thruster is an isentropic expansion. In addition, the momentum conservation equation shows there is a polytropic relationship between the plasma potential and the electron density with a γ smaller than that for an atomic gas due to ionization.